In a co-injection molding apparatus, two or more molten materials are injected into the same mold cavity, either simultaneously or in sequence using a single or a plurality of injection manifolds. A typical co-injection molding apparatus comprises first and second injection manifolds that receive pressurized melt streams from respective molten material sources. Each manifold distributes a melt stream of molten material to a plurality of nozzles. The two melt streams are forced through separate channels in the nozzle and into a plurality of mold cavities. The two melt streams may enter the mold cavities simultaneously or, alternatively, the two melt streams may enter in sequence. A combination in which the melt streams first enter the mold cavities in sequence and then simultaneously may also be used. Once both materials have been deposited in the mold cavities, the melt is cooled in the mold cavities and the molded parts are released so that another cycle can begin.
Co-injection is used for example to produce food packaging products having a predetermined and very accurate amount of an inner material, such as for example oxygen barriers or having a percentage of recycled, or post-consumer material or having a percentage of a different colored material.
In general, the amount of the inner material that enters the mold cavity after injecting the first outer material must be very precise in order to produce a quality molded part. In the case of a multi cavity molding system, the quantity of the inner material must also be the same in each molded material. This inner material can be a barrier material.
It is desirable to use as much recycled material in a molded part as possible without exceeding a maximum allowable amount. As such, the amount must be measured precisely.
In order to ensure that the molded product has a consistent appearance, the amount of colored material that enters the mold cavity must be precisely measured.
In a co-injection molding apparatus, the volume of the inner or core material, such as a barrier, recycled or colored material transferred in each shot is very important. Several devices have been developed to control the volume of melt that is injected into a multi-material mold cavity, however, these devices tend to be inaccurate, difficult to operate, complex and costly to manufacture.
U.S. Pat. No. 5,223,275 to Gellert discloses a co-injection molding apparatus having two manifolds. Two separate channels are provided in a plurality of nozzles to receive material from the respective manifolds. The volumes of the first and second materials flowing into a mold cavity are controlled by the machine nozzle and therefore are not precise.
U.S. Pat. No. 5,112,212 to Akselrud et al. discloses a shooting pot, which is used as a metering device, for use in a co-injection molding apparatus. The shooting pots are remotely located with respect to the hot runner nozzle and are used to control the timing and the volume of one of the two molten materials injected into the cavity. The shooting pot includes a piston that is axially movable within a cylinder to force molten material from the cylinder into a nozzle, which leads to a mold cavity. The cylinder includes an inlet that delivers melt from a melt source to a reservoir, which is located in a lower end of the piston. The piston is rotatable to move the reservoir out of communication with the inlet to seal it off so that when the piston is lowered, a known volume of melt is forced into the mold cavity.
Other shooting pot arrangements for use in co-injection are shown in U.S. Pat. Nos. 5,143,733 and 5,200,207 and European Patent Application No. EP 0 624 449.
A disadvantage of these manifold shooting pots is that they are remotely located from the nozzle and the mold cavity and this makes the whole apparatus more space consuming. Also these shooting pots located in the manifold or adjacent the manifold include separate mechanisms located in the manifold that open and close the access of the metered molten material to the shooting pot and these mechanisms are space consuming, difficult to manufacture and hard to synchronize in a multi-cavity mold. By using these known co-injection molding devices, the measured volume of inner melt injected from the shooting pots may vary from one molding cycle to the next and from one cavity to another. This occurs because there is a large volume of melt that is located between the shooting pot and the mold cavity, i.e., the melt in the nozzle, the melt in the manifold channel and the melt in the shooting pot. This large volume of quasi metered melt introduces several process variables. Minor deviations in temperature or pressure, for example, may result in significant variations of the known volume. The sizable distance between the shooting pot and the mold cavity further causes the melt to have a long residence time outside of the nozzle between the injection of one article to the next. This results in molded parts that are not of the highest quality because the temperature of the melt coming from the shooting pot may be either under heated or over heated.
It is therefore an object of the present invention to provide a metering device for a nozzle of a co-injection molding apparatus, which obviates or mitigates at least one of the above disadvantages.